In recent years, there is a solid light source lighting device that performs a dimming of a solid light source using a LED (Light Emitting Diode) element, an organic EL (Electro Luminescence) device, or the like.
Generally, the solid light source lighting device is configured to control current flowing through solid light sources by a switching circuit (step-down chopper circuit, flyback circuit, or the like), and determine an amount of current flowing to the solid light sources with a dimming signal from a dimming signal generator, and dim the solid light sources.
For example, there is a conventional solid light source lighting device, which includes a input filter 101, a rectification circuit 102, a boost chopper circuit 103, a step-down chopper circuit 104, and a control circuit 105 as shown in FIG. 10.
The input filter 101 is configured to receive electrical power from a commercial power supply PS100, and remove an unnecessary frequency component such as a noise.
The rectification circuit 102 is configured to output a rectified voltage that is generated by performing a rectification (full wave rectification or half-wave rectification) of an AC voltage inputted via the input filter 101.
The boost chopper circuit 103 includes a series circuit of an inductor L101 and a diode D101 that are connected to a high potential side of the rectified voltage. Further, a series circuit of a switching element Q101 and a resistor R101 is connected between output ends of the rectification circuit 102 via the inductor L101. Further, a capacitor C101 is connected between output ends of the boost chopper circuit 103. Then, when the switching element Q101 turns on and turns off, a boost voltage occurs at both ends of the capacitor C101.
In the step-down chopper circuit 104, a switching element Q102, an inductor L102, a capacitor C102, and a resistor R102 are connected in series between both ends of the capacitor C101. Further, a diode D102 for regeneration is connected to a series circuit of an inductor L102 and a capacitor C102. Then, a solid light source 100 using LED elements, organic EL elements, or the like are connected between both ends of the capacitor C102. When the switching element Q102 turns on and turns off, step-down voltage occurs to both ends of the capacitor C102.
The control circuit 105 includes a boost control circuit 105a, a dimming signal conversion circuit 105b, a microcomputer 105c, and a step-down control circuit 105d. 
The boost control circuit 105a is configured to control a boost operation of the boost chopper circuit 103 by performing a switching control of on/off of the switching element Q101.
Then, a dimming signal X inputted into the control circuit 105 from the outside is a PWM (Pulse Width Modulation) signal that made duty cycle variable according to a dimming ratio. The dimming signal conversion circuit 105b is configured to convert the dimming signal X to a DC voltage according to the duty cycle, and then, output the DC voltage to an A/D conversion port of the microcomputer 105c. 
The microcomputer 105c is configured to store previously a dimming curve (dimming characteristic) in which a dimming ratio becomes low as the duty cycle of the dimming signal X is large. Then, the microcomputer 105c is configured to calculate a dimming ratio (value of output current) according to the DC voltage (equivalent to the duty cycle of the dimming signal X) inputted from the dimming signal conversion circuit 105b with reference to the dimming curve. The microcomputer 105c is configured to generate a step-down control signal S according to the calculated dimming ratio, and then output this step-down control signal S to the step-down control circuit 105d. This step-down control signal S is a signal that indicates the dimming ratio.
The step-down control circuit 105d is configured to control a step-down operation of the step-down chopper circuit 104 by performing a switching control of on/off of the switching element Q102 based on the step-down control signal S.
There are a DC dimming that makes a direct current supplied to the solid light source 100 (for example, see JP 2005-347133 A and JP 2010-205778 A), and a burst dimming that turns on and turns off a direct current and performs a PWM control (for example, see JP 2011-150878 A), as the dimming method of the solid light source 100. There is also the dimming method that combines the direct current dimming and the burst dimming (for example, see JP 2011-108668 A).
At the time of dimming, there is the case where a video flicker caused by matching with the optical power waveform of the solid light source 100 and a movie camera machine, such as a video camera. In order to suppress this video flicker, it is necessary to make small the difference of the peak value and the bottom value of the current flowing through the solid light source 100 as possible, and bring the current flowing through the solid light source 100 close to a DC wave with few current ripples.
Then, the capacitor C102 is connected to the output of the step-down chopper circuit 104, that is, the solid light source 100 in parallel. Therefore, the difference of the peak value and the bottom value of the current can be made small, and accordingly, it is possible to bring the current close to the DC wave with few current ripples (for example, see JP 2011-60615 A). In this time, a voltage Vo2 (hereinafter, referred to as an “output voltage Vo2”) across the capacitor C102 is substantially equal to the forward voltage of the solid light source 100. The forward voltage of the solid light source 100 is determined based on a dimming ratio (value of the output current) according to the forward current-forward voltage characteristic of the solid light source 100.
The conventional solid light source lighting device shown in FIG. 10 had the following problem when the forward voltage of the solid light source 100 is too high, or when the capacity of the capacitor C102 is too large.
First, as shown in FIG. 11, it is assumed that the dimming signal Xp indicating the present dimming ratio (first dimming ratio) is changed to the dimming signal Xq indicating the higher dimming ratio (second dimming ratio) at the time of lighting of the solid light source 100. In this case, the step-down control signal S outputted from the microcomputer 105c changes from the step-down control signal Sp indicating the first dimming ratio to the step-down control signal Sq indicating the second dimming ratio.
However, when the optical power of the solid light source 100 is raised from the first dimming ratio to the second dimming ratio (first dimming ratio< second dimming ratio), the charging time period of the capacitor C102 is required. Therefore, the response time period t101 is taken for raising the output voltage Vo1 from the output voltage Vp equivalent to the first dimming ratio to the output voltage Vq equivalent to the second dimming ratio (refer to Y101 in FIG. 11).
As shown in FIG. 12, it is assumed that the dimming signal Xr indicating the higher dimming ratio (first dimming ratio) is inputted at a lighting-off state of the solid light source 100 instead of the dimming signal X0 indicating turning off (dimming ratio 0%). In this case, the step-down control signal S outputted from the microcomputer 105c changes from the step-down control signal S0 indicating a dimming ratio of 0% to the step-down control signal Sr indicating the first dimming ratio.
However, when the optical power of the solid light source 100 is raised from the lighting-off state to the first dimming ratio, the charging time period of the capacitor C102 is required. Therefore, the response time period t102 is taken until the output voltage Vo2 reaches the forward voltage Vth, at which the solid light source 100 starts lighting (refer to Y102 in FIG. 12). When the dimming ratio as a target is lower than the first dimming ratio, the response time period t103 taken until the output voltage V2 becomes the forward voltage Vth becomes longer than the time t102 (refer to Y103 in FIG. 12).
That is, when the solid light source 100 lights at a dimming lower limit from the lighting-off state, the current supplied to the solid light source 100 decreases as the value of the dimming lower limit is low, and then, the charge of the capacitor C102 decreases. That is, since there are few charges of the capacitor C102, the response time period taken until the solid light source 100 start lighting or the response time period until the solid light source 100 reaches a desired dimming ratio become too long. That is, there is a possibility that the dimming ratio of the dimming lower limit is restricted depending on the maximum of the permissible response time period.